1. Field of the Invention
The present invention relates to the electronics manufacturing art and, in particular, to a method for manufacturing multichip module deposited substrate boards in large format while minimizing warpage and metal interdiffusion and corrosion at the metal to insulating material interface. With greatest particularity, the present invention relates to a low cost, low temperature, low stress method for manufacturing large format multichip module deposited substrate boards.
2. Background Information
As electronic device technologies have developed and evolved, the trend has been to ever greater levels of electronic circuit complexity and miniaturization. The rapid progress made in very large scale integration (VLSI) technology has led to integrated circuits (IC) with finer features, increased I/O pin count, greater speed and functionality, and higher power dissipation. Silicon emitter coupled logic (ECL) chips used in high performance workstations, mainframe computers, and in several supercomputers have been running with clock rates of 250 MHz. Gallium Arsenide (GaAs) VLSI and large scale integration (LSI) chips capable of operating in the GHz regime are becoming more widely available. These advances have pushed the performance of standard single-chip surface-mount and through-hole packaging technology beyond their limits.
Multichip modules (MCM), which offer shorter interconnects with controlled impedance and high wiring density to connect multiple bare die on a single substrate, have been proposed as the technology of next generation packaging. MCM-D, which is fabricated with alternating deposited layers of high density thin-film metals and low dielectric constant insulating materials, is especially suited to meet both high packing density requirements and high speed performance. MCM-D allows semiconductor die to be placed very close together to minimize delays between chips, decrease packaging capacitance and inductance 1! thus, simplifying power distribution 2!. Furthermore, with proper selection of a high heat transfer material as the substrate, MCM-D promises to enhance thermal management 2!, and to improve reliability 3!.
An MCM-D substrate typically consists of five metal levels: a ground plane, a power plane, two signal conductor layers, and a terminal metal layer. The intermetal dielectric materials used are organic polymer, such as polyimides, polyquinoline, triazine, and BCB (bisbenzocyclobutenes). The most common of these is polyimide, which because of its low dielectric constant (.about.3.1), excellent processing characteristics, and extreme resistance to temperature and chemical etchants, is a natural choice. Popular substrate materials are ceramics, silicon wafers, aluminum nitride and metals. Copper is the conductor material of choice because of its high electrical conductivity. However, very thin interface materials such as chromium, titanium, or a combination of titanium (for adhesion) and tungsten (as a barrier layer) are used to promote adhesion of copper to polyimide.
The typical fabrication approach consists of sequential deposition and patterning of conductor and dielectric materials on rigid substrates using semiconductor thin film techniques. Polyimide is applied by spin, spray or extrusion coating, and vias are defined by either reactive ion etching or a photo-imaging process. Copper is usually sputtered or plated and patterned photolithographically. By constructing MCM-D products with semiconductor processing techniques, the manufacturer can employ photolithography manufacturing processes to achieve the maximum high density interconnection (HDI). However, due to its inherent higher processing cost, MCM-D continues to remain as the packaging solution only for high-end applications. While the perceived economic constraints have inhibited widespread use of MCM-D in today's marketplace, the ability to manufacture MCM-D in large format has been extensively studied and considered as the approach to meet the future performance challenges and be cost effective 4-7!.
Even though large format fabrication is promising as an approach to low cost MCM-D, there are a number of technical difficulties which must be overcome to achieve reasonable manufacturing yield. One of the largest problems is stress induced substrate warpage. In the current copper/polyimide MCM-D fabrication process, the deposited polyimide requires a final cure at 400.degree. C. for one hour to achieve its maximum chemical stability. During the high temperature annealing step, the deposited low stress copper conductor will recrystallize resulting in relief of residual intrinsic stress introduced during the sputtering process. The elevated temperature provides the molecules with the kinetic energy required for relocation to their lowest energy position. The overall MCM-D stress drastically increases when the substrate returns to room temperature due to greater influence of the lattice mismatch between the substrate and the copper film because of the formation of long range crystal structure (the original film is basically amorphous). This phenomenon is illustrated in FIG. 1. This phenomenon repeats itself with the addition of each thin-film metal layer in the manufacturing process.
FIG. 1 shows the stress level of the copper conductor increases more than 250 times after a short one minute bake at 390.degree. C. Heat induced stress and coefficient of thermal expansion (CTE) mismatch between copper, polyimide and substrate material will cause the substrate to warp. As the MCM-D substrate size (area) increases, the substrate warpage problem becomes more severe, and leads to a multitude of problems in processing areas such as photolithographic alignment, substrate handling, film thickness uniformity in deposition and etching.
In particular, photolithographic processes can only tolerate a certain amount of substrate warpage before it becomes impossible to pattern the thin-film metal layer accurately because the warped surface no longer resides uniformly within a suitably thin focal zone (plane) and some image features will unavoidably be out of focus at some locations. For this reason, a perfectly flat substrate is ideal, and manufacturers should seek to minimize deviation from that flat substrate surface.
In addition, the high temperature curing will promote the metal interdiffusion and corrosion at the metal-polyimide interface that may adversely affect the product reliability. Furthermore, the currently proposed high temperature MCM-D fabrication process threatens to inhibit or discourage the MCM-D manufacturers from using low cost laminated substrates, and may thereby cause them to lose the competitive edge.
Therefore, it is essential to have a low temperature, low stress, and low cost approach to successfully fabricate the MCM-D in large format, e.g., 24".times.24" panels.